E-PolyLearning

36. The thermal resistance for heat conduction through a hollow sphere of inner radius r1 and outer radius r2 is
a. r₂ – r₁/4πk r₁r₂
b. r₂ /4πk r₁r₂
c. r₁/4πk r₁r₂
d. 4πk r₁r₂
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Answer: (a).r₂ – r₁/4πk r₁r₂

37. A spherical vessel of 0.5 m outside diameter is insulated with 0.2 m thickness of insulation of thermal conductivity 0.04 W/m degree. The surface temperature of the vessel is – 195 degree Celsius and outside air is at 10 degree Celsius. Determine heat flow per m² based on inside area
a. – 63.79 W/m²
b. – 73.79 W/m²
c. – 83.79 W/m²
d. – 93.79 W/m²
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Answer: (b).– 73.79 W/m²

38. The quantity d t/Q for conduction of heat through a body i.e. spherical in shape is
a. ln (r2/r1)/2πLk
b. ln (r2/r1)/πLk
c. ln (r2/r1)/2Lk
d. ln (r2/r1)/2πk
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Answer: (a).ln (r2/r1)/2πLk

39. A spherical vessel of 0.5 m outside diameter is insulated with 0.2 m thickness of insulation of thermal conductivity 0.04 W/m degree. The surface temperature of the vessel is – 195 degree Celsius and outside air is at 10 degree Celsius. Determine heat flow
a. – 47.93 W
b. – 57.93 W
c. – 67.93 W
d. – 77.93 W
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Answer: (b).– 57.93 W

40. If we increase the thickness of insulation of a circular rod, heat loss to surrounding due to
a. Convection and conduction increases
b. Convection and conduction decreases
c. Convection decreases while that due to conduction increases
d. Convection increases while that due to conduction decreases
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Answer: (d).Convection increases while that due to conduction decreases